The present invention relates to diesel engines of the type comprising:                one or more cylinders and at least one intake valve for each cylinder, provided with elastic means that push the valve towards a closed position; and        at least one camshaft for controlling the intake valves by means of respective tappets;        wherein at least one intake valve for each cylinder is controlled by the respective tappet, against the action of the aforesaid elastic means, via hydraulic means including a pressurized fluid chamber;        wherein the pressurized fluid chamber is designed to be connected, by means of a passage controlled by a solenoid valve, with an exhaust channel, so that, when said solenoid valve is open, the intake valve is uncoupled from the respective tappet and is kept closed by the aforesaid elastic means;        there being associated to said engine electronic control means for controlling each solenoid valve in such a way as to vary the time and/or the stroke of opening of the respective intake valve as a function of the operating conditions of the engine.        
Some time ago the present applicant developed a system for variable actuation of the intake valves of the engine, identified by the trademarks UNIAIR and MULTIAIR (see EP-A-803 642, EP-A-0 961 870, EP-A-0 931 912, EP-A-0 939 205, EP-A-1 091 097, EP-A-1 245 799, EP-A-1 243 763, EP-A-1 243 762, EP-A-1 243 764, EP-A-1 243 761, EP-A-1 273 770, EP-A-1 321 634, EP-A-1 338 764, EP-A-1 344 900, EP-A-1 635 045, EP-A-1 635 046, EP-A-1 653 057, EP-A-1 674 673, and EP-A-1 726 790).
FIG. 1 is a schematic illustration of the principle of operation of an electronically controlled hydraulic system for variable actuation of the intake valves of the engine, of the so-called UNIAIR or MULTIAIR type, which has been developed by the present applicant and has formed the subject of the various prior patents that have been indicated above. With reference to said figure, number 1 designates as a whole an intake valve associated to a respective intake duct 2 formed in a cylinder head 3 of an internal-combustion engine, specifically a diesel engine in the case of the present invention. The valve 1 is recalled towards its closed position (upwards as viewed in the figure) by a spring 4, whilst it is forced to open by a piston 5 acting on the top end of the stem of the valve. The piston 5 is in turn controlled by means of oil under pressure that is present in a pressurized chamber 6, acting on which is a pumping piston 7, which moves together with a tappet a that co-operates with a cam 9 of a camshaft 10. The tappet 8 is pushed by a spring 11 and is in sliding contact with the cam 9. The pressure chamber 6 is designed to be connected to an exhaust duct 12, which in turn communicates with an accumulator of pressurized oil 13, through a passage controlled by the open/close element 14 of a solenoid valve 15, which is in turn controlled by electronic control means, designated as a whole by E, as a function of the operating conditions of the engine. In the preferred embodiment of the aforesaid system, the solenoid valve 15 is of a normally open type. In said open condition, the chamber 6 is in communication with the discharge passage 12 so that the cam 9 is de-activated, since the movements of the tappet 8 and of the pumping piston 7 do not cause corresponding movements of the piston 5 for controlling the valve 1. Consequently, the latter remains in its closing position, in which it is held by the spring 4. When the solenoid valve 15 is closed, the chamber 6 is again pressurized, filling with oil coming from the passage 12 (which communicates with the circuit for lubrication of the engine) and from the accumulator 13, through an auxiliary passage controlled by a non-return valve 16, as well as through the passage of communication with the engine-lubrication circuit, controlled by the non-return valve 17. In said condition, the cam 9 is rendered active, in so far as the movements of the tappet 8 and of the pumping piston 7 are transmitted to the piston 5, which controls the movement of the valve 1. When the solenoid valve 15 is again brought into its open condition, the oil present in the chamber 6 is discharged, through the passage controlled by the solenoid valve 15, into the accumulator 13 so that the valve 1 closes rapidly on account of the spring 4, the cam 9 being thus rendered again inactive. The solenoid valve 15 is controlled by the electronic means E in the various operating conditions of the engine according to any pre-set strategy so as to vary as desired both the instant of opening of the intake valve and the instant of closing of the intake valve, as well as the opening stroke, so as to obtain an ideal operation of the engine, for example, from the standpoint of reduction of the consumption levels, or of reduction of noxious exhaust gases in the various operating conditions.